1. Field of the Invention
This invention relates to well logging tools and methods, and more particularly to a memory logging tool having a variable output radiation field intensity and variable area of the investigation for evaluating the degree of gravel packing.
2. Description of the Related Art
A vast array of tools are utilized to log oilfield wells during drilling and completion, and production phase of such wells. These logging tools obtain measurements relating to the drilling operation, wellbore condition and characteristics of the formation surrounding the wellbore. After drilling the wellbore to a desired depth, and before allowing it to produce hydrocarbons from a hydrocarbon-containing subsurface formation, the wellbore is made ready or “completed.”
The completion operations usually include, among other things, lining the wellbore with a casing made of jointed metal tubulars, filling the annulus between the casing and the well with cement, installing sand screens, and perforating the casing and the formation at selected depths across from the hydrocarbon-containing strata to allow the hydrocarbons to flow from the formation to the wellbore. The formation fluid flows from the formation into the well via the perforations because the formation pressure is greater than the pressure in the well.
The free flow of the formation fluid into the well causes sand in the formation to flow into the well at relatively high flow rates, which can erode the tubular and other equipment in the wellbore. Such other equipment includes flow control valves, sensors, safety devices usually installed in the well to control fluid production through the well and for safety reasons.
One or more metallic screens, usually referred to as sand control screens, are placed in the wellbore to prevent inflow of formation solid particles (fines). Gravel is packed between the formation or casing and the production tubing to inhibit sand flow into the production tubing. Proper gravel packing is a critical step in the completion of a well.
Numerous gravel packing methods or procedures have been developed to inject sand or proppant into the annulus between the permeable screen and the production tubing in high permeability formations. As noted above, the annular sand pack performs the function of filtering formation solid particles which migrate into the well so that they cannot plug or limit production and to eliminate the erosion effects of the produced sand, which can damage the wellbore equipment, and in extreme cases cause the loss of the well. These procedures are referred to in the oil and gas industry as Gravel Pack, Frac Pack, Water Pack, etc., each of which is designed to provide essentially the same function—to completely and tightly fill the screen/casing annulus with sand or poppant with no voids or partially packed intervals. The gravel pack depth can range from a few (10) to several thousand (1000–5000) feet. The gravel pack acts as a filter that prevents the entry of formation fines into the wellbore without restricting the flow of the formation fluids. It is thus important to determine the integrity of the gravel pack. The success of the gravel pack and the longevity of the wellbore depends upon the extent and continuity of the gravel pack within the annulus.
The effectiveness of gravel placement in the screen-casing annulus or behind the casing (such as when prepacking perforation tunnels) is normally evaluated with treatment-pressure data. Darcy's law, volumetric calculations along with treatment pressure evaluation and pressure testing methods are used to estimate the level of gravel fill, with the minimum requirement being that the sand level must extend into the blank pipe above the top of the screen. This allows for the potential future settling of the sand. A direct measurement locating the top of the gravel pack and the quality or continuity of the sand fill within the annulus is preferred. Such measurements can be utilized to improve the above-noted treatment pressure data derived estimates. The continuity or absence of significant voids within the packed annulus is best evaluated with a direct measurement. Locating the voids soon after the completion is important because such voids can not normally be detected with the pressure evaluation methods. Voids can require workover of the gravel pack, and in extreme cases, can even lead to complete failure of the well.
At present, voids in the gravel-packed screen-casing annulus are usually evaluated from data from density, neutron, gamma-tracer or pulsed-neutron logs. These logs are usually obtained by wireline logging tools, which require a separate trip into the well and are often not performed promptly after finishing gravel packing. Also, when radioactive materials are used for evaluating proppant placement, gamma measurements are affected by the background signals produced by such radioactive materials. These background signals make the conventional density and pulsed-neutron silicon-activation methods relatively ineffective.
The ability to alter output radiation intensity from a source sub is necessary to provide optimum log response in a variety of different logging environments. Control of source sub output intensity is traditionally accomplished by varying the intensity of an internal radioactive source. Because of the expense of a radioactive source and the problems associated with radioactive sources, it is problematic to have a large assortment of radioactive sources of varying intensities or Isotopes to meet the needs of varying logging environments. The major problems associated with producing several different radioactive sources are radiation safety, exposure, logistics, management, and hazardous waste. Thus there is a need for a method and apparatus that enables efficient alteration of the output radiation intensity from a nuclear source sub.